1. Field of the Invention
This invention relates to a method of and a system for measuring fine surface conditions or refractive index distribution of a work such as an optical part or the like by observing interference fringes, and more particularly to a method of and a system for measurement by interference fringes which are used for inspecting in a manufacturing step of a work such as an optical part or the like whether the surface of the work has been ground into a predetermined shape or whether the work has a predetermined refractive index distribution.
2. Description of the Prior Art
Recently with improvement in manufacturing accuracy of mechanical, optical or semiconductor parts, there is a requirement for a technique for conveniently measuring the shape of a surface to be processed with a high accuracy during manufacturing steps thereof.
In such measuring techniques, there has been known a method in which interference of light is generated between the surface to be processed and a reference surface by an interferometer and the shape of the surface to be processed is measured by analyzing the interference fringes generated by the interference of light. In such a method, it is difficult to determine whether the surface is concave or convex at each part by simply observing the interference fringes.
Conventionally, the operator pushes the work to move up and down the surface to be processed relative to the reference surface to move the interference fringes and determines whether the surface is concave or convex at each part on the basis of the direction of movement of the interference fringes, e.g., leftward or rightward, or inward or outward.
However, though the conventional method permits to determine whether the surface is concave or convex at each part with ease, it is not satisfactory in accuracy since the operator pushes the work on a support in a predetermined position and the position of the support can be shifted by the force of the operator.
When a piezoelectric transducer or the like is used to push the work, the aforesaid problem on the accuracy of the measurement may be overcome. However this approach is disadvantageous in view of the cost since a piezoelectric transducer is required for each measuring system. Further there has been known a fringe scanning method in which the interference fringes are analyzed on the basis of change in brightness in a predetermined position generated by a fine distance movement of the work. However the fringe scanning method requires a large apparatus and providing such large apparatuses for various manufacturing steps is disadvantageous from the viewpoint of both the cost and the space.
Further, plastic lenses have been recently put into wide use. When plastic lenses are produced, resin orientation distribution fluctuates according to flow of the molten resin and in some lenses, density distribution (refractivity) can vary from part to part. Accordingly, molded plastic lenses must be screened on the basis of a predetermined standard. Measurement by interference fringes can be employed for the screening. Further there has been proposed a method of measuring refractive index distribution of gradient index lenses or crystal growth substances by interference fringes. In such a measurement, though the absolute values of refractive index differences can be easily detected, it is difficult to determine in which direction the refractive index increases or decreases.